Generally, a manhole is mounted in openings of a drainage pipe for drainage of sanitary sewage or storm sewage, or openings of underground facilities installed for buryng electric power lines or communication lines. Referring to FIG. 1, the conventional manhole 1 comprises a manhole frame 2 located in openings to access to the sewer or the underground facilities to be buried and installed in the ground surface and a manhole cover 4 rested on a retaining step 3 formed on an inner circumferential surface of the manhole frame 2 for opening or closing the manhole 1.
Meanwhile, in case of the manhole 1 described above, since the manhole cover 4 is merely rested on the manhole frame 2, there occurs a problem in that the manhole cover 4 is separated from the manhole frame 2 without any previous warning. For example, as shown in FIG. 1, in case of the manhole 1 installed on a vehicle road, the manhole cover 4 is separated from the manhole frame 2 by a vehicle running at a high speed, and hence, there occur problems such as deviation of the vehicle from the vehicle road or an accident of collision with the following vehicles.
In addition, as shown in FIG. 2, in case where the manhole 1 is mounted on an opening of a sanitary sewer pipe 5 buried in the waterside of a river, if the manhole 1 is submerged in the water due to the flood of a river in rainy weather, the manhole cover 4 is opened by a flow speed of the river or rainy water is flowed into the sanitary sewer pipe 5 through a gap between the manhole frame 2 and the manhole cover 4, both of which results in an increase in volume of the sewage flowed into a sewage treatment plant, to thereby create an unnecessary cost for sewage treatment.
In order to solve the above problems, a variety of fastening means for preventing separation of the manhole cover from the manhole frame and manholes for maintaining watertightness have been devised. However, the fastening means or manholes are so far impossible to be put to practical use due to their structure unsuitable for casting process in spite of most manholes made of cast-ironed material, or due to in-convenience in use and troublesome problems in that the manhole cover is, once locked, hardly opened and the like as well as the complicated configuration.